Time to call a roofer. It sounds like the previous owner thought they had the problem taken care of and had finished up with painting it. Unless you think they purposely tried to defraud you or didn't provide proper disclosure this is just one of the joys of home ownership. Skylights are notorious for leaking especially if the caulk or sealant used on the seams is not the specialized kind that is used for outdoors. Doesn't sound like an expensive fix. Let me know if you need a roofer recommendation.
Typically, a buyer will only have recourse with the seller if the seller knowingly failed to disclose latent defects (defects that a buyer or inspector couldn't readily see -- like a leaky basement, etc). Even then, it would be your responsibility to prove that they knew (good luck). In this case, it sounds like the seller knew nothing about the leak. If they didn't know about it, they couldn't possibly disclose it.
Welcome to the joys of home ownership! I'm currently dealing with a water leak from an upstairs bathroom, so I know the feeling.
With two generations of owners involved, u will have tremendous amount of legal costs involved.
U might be better of just get fixed and keep the receipts for next owner.
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